Among all known materials, metals are very widely used because of certain advantageous properties: high level of mechanical resistance, high level of toughness, formability, electrical conductivity, etc. Moreover, their characteristic brilliance or lustre is very much sought after for decorative applications. Their hardness is however inferior to that of ceramics, which may result in significant wear in numerous common conditions of use, for example when these materials are used to make exterior parts used in watchmaking, such as watch cases or wristbands.
For this reason, it has long been a goal to increase the hardness of metals. The conventional metallurgic methods (quenching, structural hardening) do not however enable the very high levels of hardnesses, i.e. greater than 1,000 HV (Vickers hardness) required for certain applications, such as cutting tools or scratchproof decorative parts.
For these applications, the hardness of metals has been increased by adding very hard particles, i.e. by making two-phase composites formed of a metallic phase coating hard particles, which frequently take up most of the volume of the material. A certain number of materials of this type have been developed and are today commonly used in the industry. They are generally called "hard metals" when the hard particles used are for example tungsten carbide, and "cermet" when they are made of aceramic material, for example titanium carbonitride.
All these materials have in common however the fact that they include a distinct metallic phase which may limit their resistance to corrosion and their mechanical properties at high temperatures.
Moreover, the metallic phase used as binding agent usually contains nickel or cobalt, all attempts to substitute other metals for these elements having led to insatifactory results until now. However, the use of this metallic phase with nickel and cobalt, in applications which result in prolonged contact with the skin, such as for example for watchmaking and jewellery applications, must be avoided since these elements are known to frequently cause allergies.
Finally, these materials are generally difficult to manufacture, which constitutes a significant limitation for the manufacture of exterior parts for watchmaking or pieces of jewellery which often have complex shapes with resultant high cost.
Patent Document JP-A-02 167875 discloses a method for producing modified zirconia ceramics (ZrO.sub.2) for the purpose of changing the hardness of the latter and also giving thereto a black or grey-black colour. In order to do this, it is proposed to place a previously heated zirconia ceramic part into a reaction vessel in which plasma is generated, preferably, a microwave plasma from mixture of gases including hydrogen and a hydrocarbon gas or carbon monoxide so that the superficial layer of the ceramic material is converted into a form of zirconium carbide (ZrC) of black or grey-black appearance. The ceramic material thus modified is then used for making cutting tools, and machine parts.